Ultrasonic bond control apparatus

ABSTRACT

A method and apparatus for improving the quality of the bond obtained from an ultrasonic welding apparatus. The formation of the bond is detected by monitoring the voltage applied to the ultrasonic transducer and by detecting a drop in the applied voltage indicative of bond formation. The bonding apparatus is turned off immediately following bond formation to prevent further ultrasonic energy from weakening the bond.

United St Spanjer 1 Jan. 8, 1974 [54] ULTRASONIC BOND CONTROL 3,380,1504/1968 Daniels 228/1 X APPARATUS FOREIGN PATENTS OR APPLICATIONSInventor! Keith p i h Scottsdale, AriZ- 275,702 6/1970 U.S.S.R 228/1[73] Assignee: Motorola Inc., Franklin Park, Ill.

. Primary Examiner-J. Spencer Overholser [22] Filed 1972 AssistantExaminen-Robert J. Craig [21] Appl. No.: 240,404 Attorney-Vincentl.Rauner and Henry T. Olsen [52] U.S. Cl 228/1, 29/470.1, 73/702, [57]ABSTRACT I t C] 73/ A method and apparatus for improving the quality of58] .id 228M 8 9. the bond Obtained from an ultrasonic welding pp o earc7O tus. The formation of the bond is detected by monitormg the voltageapplied to the ultrasonic transducer Y and by detecting a drop in theapplied voltage indica- [56] References cued tive of bond formation. Thebonding apparatus is UNITED STATES PATENTS turned off immediatelyfollowing bond formation to 3,153,850 10/1964 Worlton et al 29/47l.l Xprevent further ultrasonic energy from weakening the 3,l58,928 12/1964Prisco et al....... 29/470.l bond 3,212,695 10/1965 MacGregor 29/470.l X3,302,277 2/1967 Pruden et a1. 228/1 X 6 Claims, 3 Drawing Figures 1 ULTRASO/V/C W lli $0065? POWER SUPPLY l' 27 E ENVELOPE AMPL/TUDE POWER 0ETECTOP SEA/50f? SW/ TCH PMENTEDJ 3.784.079

20 5 26 UL 77-?A50/V/C l6 man/500cm POWER SUPPLY /4 27 ENVELOPE AMPLTUDE POM/EH Q DETECTOR SENSOR SWITCH ULTRASONIC BOND CONTROL APPARATUSBACKGROUND and mechanical connections between a semiconductor device andits package are known. One such system utilizes ultrasonic weldingtechniques wherein a wire, generally of gold or aluminum, is welded to abonding pad, also generally of gold or aluminum, by bringing the wireand the bonding pad together under pressure and moving the wire withrespect to the bonding pad for a predetermined time duration at anultrasonic rate until sufficient heat is generated to weld the wire tothe bonding pad. The welding cycle is timed, the time being adjusted inaccordance with the diameter of the wire used and the amount of pressureapplied. The time duration ofthe cycle is adjusted to form a bond underaverage conditions.

Another such system employs a transducer to sense the motion of thebonding head, and to turn off the apparatus when the amplitude of thevibrations of the ultrasonic head is reduced, the reduction beingindicative of a bond formation.

Whereas these techniques provide a way to achieve a connection between asemiconductor device and its package, the time required for the bond toform is not readily predictable due to variations in surface properties.Therefore, the first technique requires that the ultrasonic transducerbe operated for a relatively long period oftime to assure that bondingoccurs. However, in cases where the bond occurs rapidly, the continuedapplication of ultrasonic energy weakens the bond and reduces thereliability of the device. The second tech' nique provides a way toreduce the bond weakening problems associated with the first technique,however, the accuracy of this technique in detecting bond formation islimited.

SUMMARY It is an object of the present invention to provide an improvedwelding apparatus that consistently provides good quality bonds.

It is a further object of this invention to provide a welding apparatusthat automatically terminates the welding process when an optimum bondis achieved.

It is another object of this invention to provide a welding apparatusthat automatically adjusts the welding time to compensate fordifferences in the materials being welded.

In accordance with a preferred embodiment of the invention, a wedge isused to apply ultrasonic welding energy to the work-piece. The wedge iscoupled to a transducer by means of a horn comprising discrete diametersections. Ultrasonic energy is applied to the transducer from a powersupply to excite the wedge. A

sensing circuit monitors the power applied to the transducer and shutsoff the ultrasonic power supply when a relatively sharp power change,resulting from a change in acoustic impedance during bond formation,

occurs.

DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 shows, in block diagram form, a system for improving the qualityof ultrasonic welds according to the invention, as applied to anultrasonic welding apparatus;

FIG. 2 shows a waveform of the voltage appearing at the electrical inputterminals of the transducer of FIG. 1 during a typical welding cycle;and

FIG. 3 shows the envelope of the waveform of FIG. 2 which is obtained atthe output of the envelope detector of FIG. 1.

DETAILED DESCRIPTION Referring to FIG. I, which shows a diagram of apreferred embodiment of the invention, a bonding head 5 comprising awedge 10 and a horn 20 applies pressure and ultrasonic energy to aworkpiece, in this embodiment a wire 12, which is to be bonded to asecond workpiece, in this embodiment a semiconductor bonding pad I4,which is attached to a semiconductor chip 16. A support structure 18 isused to support chip 16 and bonding pad 14 during the welding operation.The aforementioned structure may be of conventional design.

Wedge 10 is driven by an ultrasonic transducer 25 which receiveselectrical energy via connections 26 and 27 and applies acoustic energyto wedge 10 through an acoustic energy transfer means, in thisembodiment, horn 20. Horn 20 may be ofa conventional design used inultrasonic welders, however. in a preferred embodiment, the horncomprises a series of constant radius cylinders coupled to each other toform a tapering structure having a series of discrete radii. Anultrasonic power supply 30, which is of conventional design, isconnected to transducer 25 and provides ultrasonic al ternating currentpower to energize transducer 25.

An automatic shut-off system, according to the invention, comprises, inthis embodiment, an envelope detector 35, an amplitude sensor 40 and apower switch 45, which are connected to each other, to transducer 25 andto ultrasonic power supply 30, as illustrated in FIG. 1. The automaticshut-off system de-activates power supply 30 when the bond between wire12 and bonding pad 14 occurs.

The formation of the aforesaid bond is evidenced by a drop in thevoltage at the power input point of transducer 25 at connections 26, 27.Referring to FIG. 2, there is shown a waveform of the voltage appearingat the input connections 26, 27 of transducer 25 during a typicalwelding cycle. The dotted line in FIG. 2 is used to indicate theenvelope of the waveform. The welding cycle begins at point when powerin the form of an alternating current voltage having a frequency of 50to KHZ is applied to transducer 25. The voltage builds up as the weldingprocess begins until a maximum value determined by the setting of powersupply 30 is achieved at point 52. During this time, acoustic energy istransferred to wedge 10 by horn 20 causing wedge 10 to vibrate at anultrasonic rate and to move wire 12 with respect to pad 14 along thesurface of pad 14 at the ultrasonic rate. As the wire 12 is movedrelative to pad 14, the heat generated by sheer stress and frictioncauses a portion of wire 12 to become plastic and to eventually adhereto pad 14. We believe that as the wire becomes plastic and a bond ismade, the motion of wedge is momentarily arrested when the wire bonds topad 14 and no longer moves with wedge 10. The mechanical load change istransferred via horn to transducer and appears as a voltage change atthe electrical connections 26, 27 of transducer 25. The mechanical loadchange results in a relatively sharp change in the power drawn bytransducer 25, thereby causing dip 54 in the envelope of the waveform ofFIG. 2 at the instant of bonding. After the bond has been made, wire 12is firmly attached to pad 14 and no longer moves with wedge 10, and theload caused by wedge 10 moving with respect to wire 12 is transferred totransducer 25 and is indicated by the flat region 56. The amplitude ofthe envelope in region 56 is different from the value of the envelope atpoint 52 and is generally lower than the value of the envelope at point52. It should be noted that although the voltage being monitored is thedriving voltage at connections 26, 27, any voltage indicative of theload on wedge 10, such as, for example, voltage appearing on a secondarywinding on an electromagnetic transducer may be monitored and still fallwithin the scope of the invention.

The elapsed time between initiation of the bonding cycle at point 50 andthe time at which the weld occurs at region 54 is a function of thecondition of the surfaces of wire 12 and pad 14, the voltage applied totransducer 25, the pressure applied to wedge 10 and the diameter of wire12. The time required for a bond to take place varies widely due tosurface conditions of pad 14 and wire 12 even when the pressure appliedto wedge 10, the diameter of wire 12 and the voltage applied totransducer 25 are closely controlled. For example, for typical wirediameters, wedge pressure and transducer voltage, the time required fora bond to occur ranges from 10 to 15 milliseconds when the surfaces ofwire 12 and pad 14 are exceptionally clean to over I50 milliseconds whenthe surfaces are dirty, with bonding times in the range of 30 to 40milliseconds being typical.

According to the prior art, in order to assure that most attempts resultin a good weld, transducer 25 is generally energized for a time periodof 40 to 50 milliseconds to assure proper bonding of wires and padshaving typical surfaces. Unfortunately, the continued operationoftransducer 25 after a bond has been made causes wedge 10 to vibratewith respect to wire 12, thereby deforming and weakening wire 12 and thebond between wire 12 and pad 14. Hence, in apparatus operating accordingto the prior art, the bonds that occur early in the welding cycle areweakened by subsequent vibration of wedge 10, and bonds that requiremore than 40 to 50 milliseconds to form, do not form.

A bond detection system according to the prior art, which attempts tominimize the bond weakening problems of the aforementioned system,utilizes bonding head amplitude sensors or transducer current sensors tosense the difference in acoustic impedance at wedge 10 between thepre-bonding condition wherein wire 12 moves with wedge 10 and. the postbonding condition wherein wire 12 is bonded to pad 14 and wedge 10 moveswith respect to wire 12. This difference correerably, there being littleor no change between points 52 and 56 in some devices.

The sensing function is accomplished by envelope detector 35, amplitudesensor and power switch in this embodiment. It should be noted, however,that any circuitry performing this function may be used and still fallwithin the scope of the invention. Envelope detector 35 is connected totransducer 25 to detect the voltage appearing at the electricalconnections 26, 27 of transducer 25. Envelope detector 35 may be ofconventional design, and may include a simple diode detector. The outputsignal from envelope detector 35 has a voltage that is proportional tothe amplitude of the envelope of the signal appearing at the input totransducer 25.

A typical waveform of the voltage appearing at the output of envelopedetector 35 is shown in FIG. 3. Re-

gions a, 52a, 54a and 56a of FIG. 3 correspond in time to regions 50,52, 54 and 56 of FIG. 2, respectively. The output signal from envelopedetector 35 is applied to amplitude sensor 40, which may be any circuitcapable of detecting a drop in voltage, such as, for example, adifferential amplifier having the signal from envelope detector 35applied to one input and a reference voltage less than the voltageoccurring in region 520 but greater than the minimum voltage of the dip54a applied to the other input. Amplitude sensor 40 generates a signalin response to the voltage drop at 5411 for triggering power switch 45,which may include, for example, a solid state switch or a relay forturning off ultrasonic power supply 30.

In order to assure that dip 54a is a well-defined dip, care must betaken in the design of horn 20. If this is not done, the change inacoustic impedance occurring at wedge 10 at the moment that the bondoccurs, will not be effectively transmitted to transducer 25, therebycausing the dip to be very slight, or causing several dips to occur dueto reflections within horn 20. It has been found through experimentationthat a continuously tapering horn does not provide a suitable couplingbetween wedge 10 and transducer 25 for clearly defining the change inacoustic impedance at the instant of bonding, and that the change inacoustic impedance is more effectively transferred to transducer 25through the use ofa horn comprising several constant radius cylindricalsections connected to each other to form a horn having several discreteradii rather than a continuous taperv Other configurations havingdiscrete steps in the taper have also proven effective. The discretesteps minimize the reflection problem caused by continuously taperedhorns and produce a well defined voltage drop at the input of transducer25 when bonding occurs. I

The techniques of the instant invention provide a means for determiningwhen an ultrasonic bond is formed by detecting a voltage change at theultrasonic transducer that results from a change in the loading on thebonding head at the instant the bond is formed. This allows theultrasonic welding apparatus to be turned off immediately following bondformation, thereby preventing weakening of the bond by subsequentapplication of ultrasonic energy, and provides more reliable ultrasonicbonds than could be heretofore achieved.

I claim:

1. In an acoustic bonding apparatus utilizing an electrically drivenultrasonic transducer and a bonding head energized by said transducer, asystem for detecting the formation of a bond, comprising, means coupledto said transducer for monitoring the power supplied to said transducerand for detecting a relatively sharp change therein, said relativelysharp change being indicative of the formation of said bond, and switchmeans connected to said monitoring means and responsive thereto forde-energizing said head following said relatively sharp change.

2. In an acoustic bonding apparatus utilizing an ultrasonic transducerhaving an electrical connection thereto, and a bonding head energized bysaid transducer, a system for detecting the formation of a bond,including in combination, means coupled to said transducer formonitoring the voltage appearing at said elec trical connection and fordetecting a drop in the amplitude thereof, said drop being indicative ofthe formation of said bond, and switch means connected to said sensingmeans and responsive thereto for de-energizing said head following saiddrop in said voltage.

3. A system as recited in claim 2 further including acoustic energytransfer means coupled to said transducer and to said bonding head forcoupling acoustic energy to said bonding head and for transferring achange in acoustic impedance to said transducer, said impedance changeresulting from an impedance change'occurring at said bonding head uponformation of said bond.

4. A system as recited in claim 3 wherein said energy transfer meansincludes a horn having means for reducing reflections of acoustic wavestherein.

5. A system as recited in claim 3 wherein said energy transfer meansincludes a horn having discretely different cross sectional areas alonga longitudinal axis.

6. A system as recited in claim 5 wherein said horn comprises aplurality of cylindrical structures having different radii coaxiallycoupled to each other to form a horn having discrete steps.

1. In an acoustic bonding apparatus utilizing an electrically drivenultrasonic transducer and a bonding head energized by said transducer, asystem for detecting the formation of a bond, comprising, means coupledto said transducer for monitoring the power supplied to said transducerand for detecting a relatively sharp change therein, said relativelysharp change being indicative of the formation of said bond, and switchmeans connected to said monitoring means and responsive thereto fordeenergizing said head following said relatively sharp change.
 2. In anacoustic bonding apparatus utilizing an ultrasonic transducer having anelectrical connection thereto, and a bonding head energized by saidtransducer, a system for detecting the formation of a bond, including incombination, means coupled to said transducer for monitoring the voltageappearing at said electrical connection and for detecting a drop in theamplitude thereof, said drop being indicative of the formation of saidbond, and switch means connected to said sensing means and responsivethereto for de-energizing said head following said drop in said voltage.3. A system as recited in claim 2 further including acoustic energytransfer means coupled to said transducer and to said bonding head forcoUpling acoustic energy to said bonding head and for transferring achange in acoustic impedance to said transducer, said impedance changeresulting from an impedance change occurring at said bonding head uponformation of said bond.
 4. A system as recited in claim 3 wherein saidenergy transfer means includes a horn having means for reducingreflections of acoustic waves therein.
 5. A system as recited in claim 3wherein said energy transfer means includes a horn having discretelydifferent cross sectional areas along a longitudinal axis.
 6. A systemas recited in claim 5 wherein said horn comprises a plurality ofcylindrical structures having different radii coaxially coupled to eachother to form a horn having discrete steps.